6.2v battery with Pentax 67?

I'm (still) trying to figure out why the mirror sticks on my Pentax 67, which I'd sent off to Asahi Photo LTD in London and then again when the problem persisted. The invoice states 'Mirror box/shutter overhaul' and I find it perplexing that the problem wasn't fixed. I was told on the phone that he'd ran several rolls of film through the camera (ten I think he said - I'm sure dummy rolls) and the mirror hadn't stuck for him. For me, it sticks almost without exception on the first frame, then randomly, usually at least twice more throughout a roll.

Something that I'd thought nothing of until now is that I'm using Energizer 544 Silver Oxide 6.2v batteries. Could this .2v be keeping the mirror up? The mirror is released eventually, after turning the speed dial several times, but released straight away when taking the battery out. Most threads online only list 6v batteries for the 67, so I'm wondering if that tiny bit of extra voltage is holding the mirror in place. I don't know the first thing about electronics, but when I bought the batteries I'd assumed an extra .2v would be negligible, but perhaps this is only the case with certain devices/cameras...

However, as I'd left the battery in when sending the camera for repair, it seems strange that he would take it out and use his own. I still don't understand why the mirror hadn't stuck for him.

Shutter sticking too?

Both the shutter and mirror stay up. The frames are 'lost' due to overexposure.

I've sent an email to Eric Hendrickson who told me to clean the release selenoid. I'd like to open up the camera myself - the quandary has become irritating and I feel like a helpless housewife with a broken hoover. If anyone could advise about taking the 67 apart and, since I'm expecting a whole universe of mechanisms in there, where exactly the selenoid is?

From what you just said I conclude that it is no good idea to attempt to repair it yourself.
Even having quite some expertise on fine-mechanics did not save me from spoiling some things. My advice is to give it to a repair shop. I understand though that with your camera twice having been at a repair shop in vain does not encourage you to try that again.